This invention relates to a circuit arrangement for igniting and operating a discharge lamp, which arrangement comprises a DC-AC converter provided with
a branch A provided with terminals suitable for connection to a DC voltage source and comprising at least one switching element for generating a current with alternating polarity by being conducting and non-conducting at a frequency f, PA1 a load branch B comprising inductive means, capacitive means, and means for coupling the discharge lamp to the load branch B, and PA1 a control circuit for rendering the switching element conducting and non-conducting at the frequency f and comprising a resonant circuit which comprises further inductive means and further capacitive means. PA1 a branch C coupled to the resonant circuit and comprising a series arrangement of a frequency-dependent impedance and a semiconductor element provided with a control electrode for influencing the impedance of the semiconductor element dependent upon a potential at the control electrode, and PA1 a branch D coupled to the load branch and to the control electrode of the semiconductor element for influencing the potential of the control electrode in dependence on the voltage across the discharge lamp.
Such a circuit arrangement is known from European Patent Application 442572A1 which corresponds to U.S. Pat. No. 5,142,201 (Aug. 25, 1992). The known circuit arrangement is in particular designed for electrodeless low-pressure mercury discharge lamps and is so dimensioned that the operating frequency f of the circuit arrangement lies above the resonance frequency of the load branch both during stationary lamp operation and during ignition of the discharge lamp in order to limit power dissipation in the switching element. Ignition of the discharge lamp and stable lamp operation often take place at a substantially constant value of the frequency f. The amplitudes of the voltages and currents present in the circuit arrangement during ignition of the discharge lamp are often considerably higher than during stable lamp operation. Since these comparatively high voltages and currents can strongly reduce the life of the circuit arrangement, especially if the lamp does not (immediately) ignite as a result of, for example, ambient factors, it is desirable to provide the switching arrangement with means which prevent the amplitudes of the voltages and currents in the circuit arrangement reaching excessive values. These means may, for example, comprise voltage-limiting elements coupled to the load branch which become current-conducting when the amplitudes of the voltages and currents in the circuit arrangement assume excessive values, thus reducing the resonance frequency of the load branch. Since the operating frequency f remains substantially unchanged, the difference between the operating frequency and the resonance frequency of the load branch increases, so that the amplitudes of voltages and currents in the circuit arrangement decrease. It was found, however, that these voltage-limiting elements must comply with particularly high requirements, as a result of which they must be assembled from comparatively expensive components and nevertheless have comparatively short lives.